1. Field of the Invention.
This invention relates to an active suspension system and the use of piezoelectric materials and controls on conventional suspension systems. More particularly, the invention relates to automotive active suspension systems using piezoelectric elements to change the stiffness and damping characteristics of the suspension systems.
2. Description of the Prior Art.
Two areas of prior art that are relevant to the present invention are piezoelectric materials and automotive suspension systems. These areas of prior art are discussed below.
PIEZOELECTRIC MATERIALS: Materials that generate electric potentials when subjected to mechanical stresses are called piezoelectric materials. When subjected to an electrical field, piezoelectric materials undergo mechanical strains and will change dimensions. Piezoelectric materials that vary dimensions in expansion or contraction upon the application of an electric field have long been used as motive elements. Piezoelectric materials also exhibit polarity effects, in that reversing the polarity of an applied electric field reverses the dimensional changes of the materials.
An example of a piezoelectric device utilizing the properties of dimensional change and polarity effects is U.S. Pat. No. 4,349,183 to Wirt et al., which discloses the use of piezoelectric crystals bonded to a flexure spring assembly in a ring laser gyro dither mechanism. The spring assembly connects a hub to a ring concentric with the hub. Application of a voltage of alternating polarity to the crystals causes the crystals to flex in a first direction, and then to flex in an opposite direction as the polarity of voltage is reversed. The flexing crystals cause the attached spring assembly to flex similarly, and consequently cause the hub to take on an oscillating rotational movement relative to the ring.
U.S. Pat. No. 4,849,668 to Crawley et al. discloses the use of a composite structural member formed of multiple layers of graphite fibers bound within an epoxy matrix. A plurality of piezoelectric sensors detect displacement of the member. The sensors communicate with a controller and the controller generates electrical excitations which are applied to piezoelectric elements embedded within the member to induce displacement of the member.
The dimensional changes of piezoelectric materials can be rapidly and precisely controlled. The response time of piezoelectric materials to any external loading is 10 milliseconds or less. As an actuator, piezoelectric materials offer controllable, precise displacements in the micrometer range. Although the displacement of a flexing piezoelectric crystal is small, piezoelectric materials can exert significant surface forces. Piezoelectric materials produce little heat or noise during operation and so operate with high efficiency.
Piezoelectric materials are both naturally occurring and man-made. Anisotropic materials such as quartz, tourmaline, and rochelle salt are some of the naturally occurring piezoelectric materials. Synthetic piezoelectric materials are termed piezoceramics. Lead Zirconate Titanate (PZT), Lead Magnesium Niobate (PMN), and Poly-vinylidene Fluoride (PVDF) are some commercially available piezoceramics. Since piezoceramic materials can sense mechanical strain conditions and respond quickly to command excitations exerting relatively large forces, their use is being considered for active vibration control of flexible structures.
AUTOMOTIVE SUSPENSION SYSTEMS: Automotive suspension systems can be divided into three different classes:
1. Passive--no means are provided to actively control the suspension system damping or stiffness during operation.
2. Semi-active--the suspension system damping is actively controlled during operation, but the suspension system stiffness is not. PA1 3. Active--both the stiffness and the damping in the suspension system are actively controlled during operation. Active control of the suspension system results in superior ride quality, countering many undesirable aspects of a vehicle's ride, such as acceleration squat, brake dive, vehicle pitching at bumps and pits, and outward roll of the vehicle at corners.
Design considerations such as vibration isolation, space constraints, stability, reliability, and handling requirements of the application dictate the choice of a suspension system for a particular application.
The small deflection capability of piezoelectric materials has previously limited the use of piezoelectric materials in automotive suspension system applications. Typical applications have included control actuators requiring only limited movement, and pressure sensors.
For example, U.S. Pat. No. 5,013,955 to Hara et al. discloses the use of piezoelectric sensors and piezoelectric actuators to effect a semi-active suspension system by varying the damping force of a shock absorber. A piezoelectric sensor generates signals proportional to the force applied to the shock absorber. The sensors feed the signals to an electronic control unit, which generates appropriate high voltage signals for a piezoelectric actuator. The actuator moves through a limited distance to control oil flow in the shock absorber, thereby regulating the damping effect of the shock absorber.
While the semi-active suspension system of Hara et al. uses piezoelectric materials to regulate the damping in the system, the suspension system does not have all the advantages of an active suspension system. Presently, active control of a vehicular suspension system is achieved by replacing conventional springs and shock absorbers with hydraulic actuators. Johnson, Lotus Sees GM Leading With Active Suspension, Automotive News, Sep. 11, 1989, at 44. Sensors detect the movements of the suspension system and the load applied to the suspension system. The sensors feed indicative signals to an on-board computer. Other sensors feed the computer information on driving conditions such as speed and steering angle. A hydraulic power unit supplies pressurized oil to a hydraulic control unit. Upon appropriate commands from the on-board computer, the hydraulic control unit directs the pressurized oil to the hydraulic actuators at each of the wheels. The control unit regulates the pressure of the oil supplied to control both the stiffness and the damping of the suspension system.
The primary draw-back in using a hydraulic system for an active suspension system is its excessive weight, which detracts from a vehicle's fuel economy. U.S. Automobile manufacturers are facing challenges to comply with federal guidelines for improved Company Average Fuel Economy (CAFE). Hence, those in the art continue to search for a lightweight active suspension system for automobiles. It should be understood that the term "automobiles" is meant to include all vehicles with suspension systems, including trucks, motorcycles, trailers, motorhomes and other wheeled vehicles.
A more effective and lightweight active suspension system may be realized by using piezoelectric material to directly control both the stiffness and damping of the suspension system. Piezoelectric regulation of an active suspension system enables control of the vehicle in any desired ride mode, while overcoming the weight handicap of the available hydraulic systems. Additionally, it is believed that such a system will be quieter, and have a lower power consumption than hydraulic systems. Finally, the excitation of a piezoelectric suspension system can be changed more rapidly than the pressure applied to a hydraulic actuator can be changed. A piezoelectric regulated system will therefore be able to respond to a greater number of ride disturbances per second than a hydraulic active suspension system.